The purpose of this study was to test for differences in physeal structure and biomechanical outcomes after bioenhanced ACL repair using absorbable or nonabsorbable sutures. First, we found evidence for more growth plate disruption in the nonabsorbable suture group (Ethibond), which had larger tunnels, and lower BMD within the tunnels at the 15-week time point compared with the absorbable group (Vicryl). We also found that the use of nonabsorbable suture material produced significantly improved outcomes in structural properties in regard to the yield and failure loads and a trend for a lower failure displacement than the use of absorbable sutures. The fact that all sutures were disrupted at the time of biomechanical testing rules out that these differences in testing are attributable to the suture material itself but rather supports the assumption of an underlying difference in biologic response. However, the ruptured sutures might very well have influenced the amount of stress shielding that had occurred, but we cannot say how great this effect may have been as it is unknown at what time point the sutures broke.
Work by prior investigators for physeal injury after surgical interventions has shown that the most persistent physeal injuries occur when a foreign material is placed across the physis. Factors associated with increased risk of physeal malfunction in prior animal studies have included posterior tunnel placement,33,34
a high ratio of tunnel diameter to physeal surface area,35–37
excessive graft tensioning,38
incomplete tunnel filling by the graft,39,40
and graft fixation across the physis.41
In human patients, the vast majority of growth disturbances and angular deformities have been associated with graft fixation devices or bone-plugs leading to bony bars across the lateral distal femoral physis (54% of angular deformities) or epiphysiodetic effects of fixation devices crossing the tibial physis (27% of angular deformities).42
In this study, with our relatively small ratio of tunnel diameter to physeal surface area, lack of excessive tensioning, and avoidance of fixation devices across the physes, we did not see any physeal disturbances, as in accordance with prior animal and clinical studies. In contrast, the incomplete tunnel filling seen in both suture groups did not appear to have a significant effect on premature physeal closure in this study.
We found significant differences in yield (157 ± 93 N for absorbable v 194 ± 120 N for nonabsorbable) and failure loads (173 ± 101 v 211 ± 122 N), in favor of nonabsorbable sutures at both time zero and 15 weeks. However, over time the gap between the 2 groups widened. We hypothesize that the nonabsorbable sutures offered more mechanical protection than absorbable sutures after the ACL repair and thus resulted in a better biomechanical outcome. This hypothesis is supported by clinical studies of suture repair of tibial eminence fractures, where excellent results have been shown with various types of nonabsorbable sutures, even over pin-and-screw fixation.
Micro-CT revealed larger and wider tunnels in the nonabsorbable suture group. Although the difference in tunnel density between the 2 groups was statistically significant, it was less than 5% and thus not likely to be of clinical significance. However, a closer look at the cartilaginous growth plate showed not only larger affected areas in the nonabsorbable group but also a significantly lower mineral density in the nonabsorbable suture group and a higher tendency for bony spur formation in the nonabsorbable suture group. Similar bony bridges were found in a recent study of the effects of transphyseal ACL reconstruction in skeletally immature sheep but were not associated with growth disturbances.39
A recent article in the Journal of Orthopedic Research
offers an explanation of this dissociation of bone bridges and physeal arrest by showing that these bony bridges are not so much a precocious ossification of the growth plate but rather dislodged osteoprogenitor cells from the bone marrow.43
Although it is not unequivocally clear that these results will lead to a worse clinical outcome, they, at the very least, suggest a potentially greater injury to the growth plate.
The surgical technique as tested here in the porcine model is not yet in use clinically. However, as preclinical studies of bioenhanced ACL repair are showing promise,14
it is possible that this technique will be in clinical practice in the next several years. Therefore, determining the effect of suture selection on physeal status and healing ligament strength was important prior to starting clinical studies of this technique. The porcine model was selected for this study because of the great similarities in wound healing, knee biomechanics, and hematology between the porcine and human species.
This study focused on bioenhanced ACL repair, but its findings are also meaningful for ACL eminence fractures. In such injuries, the ACL has typically avulsed with a piece of bone from the tibial spine. Displaced injuries are usually treated with suture fixation, or screw fixation.44–46
Gan et al.47
and Tsukada et al.48
found good results with tension wires and anterograde screw fixation. A study by Eggers et al.49
compared sutures and screws and showed that suture fixation using nonabsorbable Ethibond has higher strength than screw fixation. However, until now there has been limited information on the effects of suture fixation on the physis.
Histologically, we found most of the tunnels in the nonabsorbable group filled with suture material and only little tissue in between the suture strands. This picture might change over time as tissue encompasses the sutures, but some remaining material will be present. In the absorbable group, we found no evidence of sutures and good tissue formation. The physes in the nonabsorbable groups were disrupted but the remaining sutures, whereas the absorbable group showed pivoting of the physis but no persistent defect. In those areas with sufficient space for new tissue to form, there was no difference in cell counts across the groups, suggesting no difference in the effect of the presence of the nonabsorbable sutures or the remnants of the absorbable sutures on cell growth.
Our study has potential shortcomings. We compared only 2 suture materials. These sutures were selected because they are commonly used in arthroscopic knee surgery. It cannot be ruled out with certainty that other sutures could respond differently. Nonetheless, there was a difference between the 2 suture types and hence further investigation is warranted. In addition, this study was performed in a large animal model where controlled rehabilitation is not an option. Whether a slower rehabilitation would result in a longer persistence of the intact nonabsorbable sutures, or even the absorbable sutures, and what effect that additional retention time would have require further study. Also, it should be noted that there was no “tunnel only” control group. However, prior studies, such as Shea et al.50
or Meller et al.,51
have already assessed the volumetric effect of transepiphyseal drilling. A recent systematic review published in Arthroscopy
lists the risk factors for physeal growth disturbance after ACL reconstruction.18
Finally, our model cannot fully reproduce the human situation. For example, we could not control postoperative rehabilitation in the animals. Likewise, the ACL injury was simulated with sharp transsection in the midsubstance. It is possible that a frayed disruption with associated cartilage and meniscal injury, as seen in the clinical situation, would heal differently. On the other hand, the frayed ends have a larger surface area, thus more contact, and might weave into each other, whereas the clean cuts may slide off. We followed these animals for 15 weeks, although clinical ACL studies usually follow patients for years. However, since we wanted to assess tunnel healing, 15 weeks has been found an appropriate time point to assess for permanent differences in tunnel healing, while at the same time being a reliable predictor of tissue healing in ACL repair.5